Water-absorbable resin (SAP/Super Absorbent Polymer) is a polymer gelling agent which is swellable with water but insoluble with water. The Water-absorbable resin is widely used, mainly disposably, for absorbing products such as disposable diapers, sanitary napkins, etc., and further for agriculture/horticulture water retaining agent, an industrial waterproofing agent, and the like. For such Water-absorbable resin, many monomers and hydrophilic polymers have been proposed as raw materials. Especially, water-absorbable polyacrylic acid resin in which acrylic acid and/or its salt is used as its monomer is most popular in industries because of its high water absorbing ability (Non-Patent Literature 1).
The water-absorbable resin is produced via a polymerizing step, a drying step, and if necessary, a non-dried matter removing step, a grinding step, a classifying step, a surface cross-linking step, and/or the like (Patent Literatures 1 to 5).
Meanwhile, the water-absorbable resin is required to have many functions in order to cope with functional sophistication of disposable diapers which are one major application of the water absorbable resin. More specifically, the water-absorbable resin is required to satisfy many properties such as, not only a high water absorbing coefficient, but also gel strength, water soluble content, water absorbing rate, an absorbency against pressure, permeability potential, particle diameter distribution, an anti-urine property, an anti microbial property, an impact resistance (anti-damage property), a fluidity, an deodorant property, an anti-coloring (degree of whiteness), low dustiness, etc. Therefore, many crosslinking techniques, additives, modifications in steps in the production, etc. have been proposed in Patent Literatures above or below.
Among these properties, the permeability potential is considered as a more important factor in association with a recent increase (for example, 50 wt % or more) in an amount of the water-absorbable resin in disposable diapers. Furthermore, methods and techniques for improving permeability potential against pressure and permeability potential without pressure, such as SFC (Saline Flow Conductivity, see Patent Literature 6) or GBP (Gel Bed Permeability, see Patent Literatures 7 to 9), etc., have been proposed.
Moreover, in addition to the permeability potential, the water absorbing rate is also a basic property for the water-absorbable resin. As one method for improving the water absorbing rate, a technique to increase a specific surface area in order to attain a greater water absorbing rate is known. More specifically, a technique for controlling to attain fine particle diameters (Patent Literature 10), techniques for granulating fine particles with a large surface area (Patent Literatures 11 to 13), a technique for freeze-drying a hydrogel to cause the hydrogel to be porous (Patent Literature 14), techniques for performing granulation and surface crosslinking of particles simultaneously (Patent Literatures 15 to 17), techniques for foaming polymerization (Patent Literatures 18 to 35), and a technique for post-polymerization foaming and crosslinking (Patent Literature 36), etc. have been proposed.
More specifically, as to the foaming polymerization, the following techniques have been known regarding a foaming agent for treating a monomer(s): techniques for using a carbonate (Patent Literatures 18 to 25), techniques for using an organic solvent (Patent Literatures 26 and 27), a technique for using an inert gas (Patent Literatures 28 to 30), techniques for using an azo compound (Patent Literatures 31 and 32), techniques for using insoluble inorganic powder or water-insoluble particles (Patent Literatures 33 and 34). Furthermore, a technique for polymerizing, without stirring, a slurry in which sodium acrylate fine precipitates are contained by 45 to 60 wt % with micro bubbles of inert gas (Patent Literature 35). Further, a technique for post-polymerization foaming and crosslinking (Patent Literature 36), etc. has been proposed.
These techniques described in Patent Literatures 10 to 36 etc. are successful in improving the water absorbing rate to some extent by increasing the surface area, etc. However, the improvement in the water absorbing rate is not so sufficient, and a special device or a costly raw material (a large amount of surfactant or foaming agent) is necessary. Further, these techniques described in Patent Literatures 10 to 36 etc. have a problem, for example, that they deteriorate the permeability potential (Patent Literatures 6 to 9), impact resistance (Patent Literature 37), bulk specific gravity (Patent Literatures 38 and 39), etc.
That is, the water absorbing rate and the specific surface area are, in general, in positive interaction relationship, but the permeability potential and the specific surface area are in negative interaction relationship. This makes it very difficult to improve both of the water absorbing rate and the permeability potential, which are largely dependent from the surface area.
Further, dispersion of the bubble by use of a large amount of surfactant as in Patent Literatures 28 and 29 leads to not only a cost increase associated with the surfactant but also to a lower surface tension of the water-absorbable resin, thereby causing an increase in amount of rewetting.
Further, the water-absorbable resin for absorbing products such as disposable diapers and sanitary napkins are used in combination with white pulps in many cases. Thus, for the sake of giving a sensation of cleanness, the water-absorbable resin is required to be white in color. Therefore, many modifying techniques in coloring have been proposed for the degree of whiteness for the water-absorbable resin (Patent Literature 40 to 42). However, at this moment, these techniques are again insufficient in terms of an anti-coloring agent cost, safety, complicate process, and further degree of their effect.